Generate a random Verification Code

Protected void page_load (Object sender, eventargs E)
{
// Generate the verification code
String checkcode = randletter (4 );
// Save the new verification code to the session
Session ["checkcode"] = checkcode;
// Enter the verification code
Createimages (checkcode );
}

/// <Summary>
/// Generate verification Image
/// </Summary>
/// <Param name = "checkcode"> Verification character </param>
Private void createimages (string checkcode)
{
Int iwidth = (INT) (checkcode. length * 13 );
System. Drawing. bitmap image = new system. Drawing. Bitmap (iwidth, 23 );
Graphics G = graphics. fromimage (image );
G. Clear (color. White );
// Define the color
Color [] C = {color. Black, color. Red, color. darkblue, color. Green, color. Orange, color. Brown, color. darkcyan, color. Purple };
// Define the font
String [] font = {"verdana", "Microsoft sans serif", "Comic Sans MS", "Arial", ""};
Random Rand = new random ();
// Random output noise
For (INT I = 0; I <50; I ++)
{
Int x = Rand. Next (image. width );
Int y = Rand. Next (image. Height );
G. drawrectangle (new pen (color. lightgray, 0), X, Y, 1, 1 );
}

// Output verification code characters of different fonts and colors
For (INT I = 0; I <checkcode. length; I ++)
{
Int cIndex = Rand. Next (7 );
Int findex = Rand. Next (5 );

Font F = new system. Drawing. Font (font [findex], 10, system. Drawing. fontstyle. Bold );
Brush B = new system. Drawing. solidbrush (C [cIndex]);
Int II = 4;
If (I + 1) % 2 = 0)
{
II = 2;
}
G. drawstring (checkcode. substring (I, 1), F, B, 3 + (I * 12), II );
}
// Draw a border
G. drawrectangle (new pen (color. Black, 0), 0, 0, image. Width-1, image. Height-1 );

// Output to the browser
System. Io. memorystream MS = new system. Io. memorystream ();
Image. Save (MS, system. Drawing. imaging. imageformat. JPEG );
Httpcontext. Current. response. clearcontent ();
// Response. clearcontent ();
Httpcontext. Current. response. contenttype = "image/JPEG ";
Httpcontext. Current. response. binarywrite (Ms. toarray ());
G. Dispose ();
Image. Dispose ();
}

/// <Summary>
/// Generate random letters
/// </Summary>
/// <Param name = "count"> Number of Generated letters </param>
/// <Returns> generated Verification Code </returns>
Private string randletter (INT count)
{
String allletters = ", A, B, C, D, E, F, G, H, I, J, K, L, M, n, O, P, Q, R, S, T, U, V, W, X, Y, Z ";
String [] allchar = allletters. Split (',');
String checkcode = "";
Int temp =-1; // record the previous random value. Avoid producing several identical random numbers.

// Use a simple algorithm to ensure different random numbers are generated
Random Rand = new random ();
For (INT I = 1; I <count + 1; I ++)
{
If (temp! =-1)
{
Rand = new random (I * temp * unchecked (INT) datetime. Now. ticks ));
}
Int T = Rand. Next (allchar. Length );
Temp = T;
Checkcode + = allchar [T];
}
Return checkcode;
}